The interaction of a phage encoded topoisomerase I with specific sites for DNA recombination is being studied by: 1) genetic analysis of the inhibition of this reaction by various topoisomerase I mutants and 2) biochemical analyses using attachment sites cloned onto a plasmid vector. Functional domains of the protein and recombination intermediates are being identified. Control regions of the genome of HSV-1 have been identified in the dDNA that occur as reiterated sequences in virion particles. An origin of replication and packaging sites are evident. When lambda HSV hybrid clones are transfected into eukaryotic cells with HSV helper, substantial editing of the lambda sequences is seen.